stack

A pillar of rock that forms an islet at the end of a headland.
A stack is the result of a series of events caused by erosion.
Sea caves striking a headland first carve a cave
by opening up joints in the rock. The cave gradually penetrates the headland
until an arch is formed. Finally, the top of the arch collapses and leaves
a stack separated from the rest of the headland. The process may be repeated
several times, giving a series of stacks.